Widget persistence API

Instead of persisting widgets ourselves, this PR provides the tools to allow users to persist widget states however they want.

This PR also includes some bugfixes, one of which is the removal of views on cell re-execution!!!
Based on #6664 

- [x] Try it
- [x] Include examples

@ellisonbg @SylvainCorlay @jasongrout 
So is there a `WidgetManager.set_state_callbacks` function?  I don't see it in the code above.  If not, how do I register the load and save callbacks?